The present invention is directed to power supply control integrated circuits with a maximum duty cycle clamp.
Pulse-width modulated control integrated circuits for use in switched/mode power supplies contain a myriad of functions including an oscillator, error amp, voltage reference, pulse width modulator, protection circuitry and output stage. In some instances, two functions may be served by one circuit block to minimize pin count and external circuitry.
The present invention serves the functions of clock oscillator and maximum duty cycle clamp. The oscillator is required to set up the basic switching frequency of a power supply. A maximum duty cycle clamp limits the percentage duty cycle or time ON relative to time OFF of the switching element to a pre-set maximum.
FIG. 1 is a simplified block diagram of a circuit that combines the functions of clock oscillator and maximum duty cycle control. The charging resistor 10 charges the timing capacitor 12 towards its top voltage V.sub.t. During this time period, the output 14 is enabled and under normal operation would remain on, supplying power to the load as required to maintain the proper output voltage. When the timing capacitor 12 reaches a voltage of V.sub.t, a comparator 16 closes switch 18 causing the discharge resistor 20 to start discharging the timing capacitor 12. The switch 18 simultaneously disables the output stage 14. The percentage of ON time is determined by selecting the timing components capacitor 12, resistor 10 and discharge resistor 20.
An integrated circuit is generally manufactured on a silicon chip. Manufacturing tolerance of the components in an oscillator integrated circuit generally produce a widely varying discharge current from one integrated circuit to the next. Thus, the prior art circuitry suffers from accuracy problems. These are most pronounced when used in a 50% maximum duty cycle configuration. In this mode of operation, the matching of the charging current to the discharge current is critical for both frequency and maximum duty cycle. The oscillator circuit is generally provided in a separate package from the charging resistor and charging capacitor. An external charging resistor can be provided with a very narrow tolerance. However, the internal discharging resistor in an integrated circuit generally has a tolerance of plus or minus 30% and a temperature coefficient of 2,000 parts per million. As such, these prior art oscillator circuits cannot be used to mass manufacture circuits requiring an accurate 50% maximum duty cycle configuration. A different charging resistor may be needed for each oscillator integrated circuit depending upon the precise value of the discharging resistor.
Limiting the maximum duty cycle is a very important factor in pulse width modulation when the output is used to drive an inductor since the time ON must be carefully controlled to prevent core saturation and subsequent catastrophic failure modes. Therefore, it is an object of the present invention to provide an oscillator integrated circuit which can be used repeatably in mass manufacture in combination with a single matching charging resistor value so as to produce a 50% maximum duty cycle configuration.